When using such tools it is usually necessary to maintain the working axis of the tool at a certain angular relationship (usually perpendicular) to the surface of a workpiece. In the case of a drill bit for example, the bit must be maintained at this angle throughout the entire drilling operation.
U.S. Pat. No. 3,707,043 and GB 2 053 750 both disclose an indicator device having:
a head arranged to be supported in a stable position on a workpiece surface adjacent to the position at which an element is to be driven into said surface, said head including a socket having a part-spherical bearing surface; and PA1 coupling means having a first portion for attachment to the tool and a second portion in sliding engagement with said first portion to move in a direction which is parallel to the drive axis of the tool, said second portion having a part-spherical bearing surface which is closely received in said socket to form a universal-type pivotal coupling. PA1 1. The construction of the device can be simplified, with less risk of failure due to dirt contamination. PA1 2. The visual indication is closer to the workpiece surface, so that both are simultaneously within the field of vision of the user. PA1 3. The visual indication can be more accurate. PA1 4. The cost of the device is reduced.
In both of these earlier proposals the head is coupled to a remote indicator arrangement disposed adjacent to the tool, to indicate when the drive axis of the tool is perpendicular to the workpiece surface. There is a serious risk of swarf and other debris becoming lodged in the coupling arrangement, which would seriously impair the operation of the device. In addition, the user is not able to focus simultaneously on the indicator and the workpiece surface, making the device difficult to use.
An aim of the present invention may be viewed as being to eliminate these drawbacks of the earlier devices.